Update to the WFPC2 Instrument Handbook for Cycle 9

Transcription

1 June 1999 Update to the WFPC2 Instrument Handbook for Cycle 9 To Be Read in Conjunction with the WFPC2 Handbook Version 4.0 Jan 1996 SPACE TELESCOPE SCIENCE INSTITUTE Science Support Division 3700 San Martin Drive Baltimore, Maryland help@stsci.edu Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration

2 User Support For prompt answers to questions, please contact the Science Support Division Help Desk. Phone: (410) World Wide Web Information, software tools, and other resources are available on the WFPC2 World Wide Web site: URL: Revision History Instrument Version Date Editor(s) WF/PC-1 1.0; 2.0; 2.1 October 1985; May 1989; May 1990 Richard Griffiths WF/PC April 1992 John W. MacKenty WFPC2 1.0; 2.0; 3.0 March 1993; May 1994; June 1995 Christopher J. Burrows WFPC2 4.0 June 1996 John A. Biretta WFPC2 Update June 1998 Andrew Fruchter, Inge Heyer WFPC2 Update June 1999 Stefano Casertano Send comments or corrections to: Science Support Division Space Telescope Science Institute 3700 San Martin Drive Baltimore, Maryland Copyright 1999, Association of Universities for Research in Astronomy, Inc. All rights reserved.

3 Table of Contents Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle Introduction... 1 Photometric Anomalies... 4 Charge Transfer Efficiency... 4 The Long vs. Short Photometric Anomaly... 7 The Stability of WFPC2 Photometric Calibration Field and Focus Dependence of Aperture Photometry The WFPC2 PSF: Dynamic Range and Photometry.. 13 Dynamic Range Photometry Dithering with WFPC Other HST Imaging Polarization Calibration Dark Current Increase Operating Changes The WFPC2 Clearinghouse Updates to System Efficiencies and Zeropoints WFPC2 Calibration Plan Introduction Overview Cycle 8 Overview References New WFPC2 Documentation, Instrument Science Reports Technical Instrument Reports Other Selected Documents Index iii

4 iv Table of Contents

5 CHAPTER 1 Update to the WFPC2 Instrument Handbook for Cycle 9 In This Document... Photometric Anomalies / 4 Charge Transfer Efficiency / 4 The Long vs. Short Photometric Anomaly / 7 The Stability of WFPC2 Photometric Calibration / 10 Field and Focus Dependence of Aperture Photometry / 11 The WFPC2 PSF: Dynamic Range and Photometry / 13 Dithering with WFPC2 / 15 Other HST Imaging / 17 Polarization Calibration / 17 Dark Current Increase / 18 Operating Changes / 19 The WFPC2 Clearinghouse / 20 WFPC2 Calibration Plan / 24 Updates to System Efficiencies and Zeropoints / 21 References / 45 New WFPC2 Documentation, / 46 Introduction The Wide Field Planetary Camera 2 (WFPC2) is now a mature and largely well-characterized instrument. Information already available through the WFPC2 Instrument Handbook and the HST Data Handbook is fairly complete. In this update we provide additional information obtained from recent studies of the instrument, as well as calibration plans for the 1

6 2 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 WFPC2, and a short note on present and planned imaging capabilities for HST. The major topics discussed in this document are: Photometric Nonlinearities: Charge Transfer Efficiency and Long vs. Short Anomaly: Recent tests of the photometric performance of WFPC2 have evidenced significant nonlinearities that can affect its response, especially for very faint sources (< 30 DN of signal per exposure). Measurements of the Charge Transfer Efficiency (CTE) of the WFPC2 charge-coupled devices (CCDs) have shown that the fraction of charge lost to impurities in the CCD has grown with time. The CTE loss is particularly high in images with low backgrounds, and can, in some cases, significantly affect photometry. Users who plan to observe in modes that typically produce images with low backgrounds for instance UV images, narrow-band images, and short exposures should examine the section on CTE to see how this may affect their program. Another nonlinearity, often referred to as the Long vs. Short anomaly, causes an increasing fraction of the charge to be lost for faint sources, and thus stars appearing slightly fainter in short exposures than in long exposures. Both anomalies have been extensively characterized over the past two years, and approximate correction formulae are now available that should reduce their impact to well below 2% for well-exposed sources, and to about 3% in the worst case (faint sources and low background). The Stability of the WFPC2 Photometric Calibration: The photometric throughput of the clean WFPC2 has remained fairly stable throughout its operational life. However, the rate at which contaminants increase after each decontamination - and thus at which UV throughput is lost - has changed over the years, mostly decreasing due to the cleaner environment of the instrument. We detail the major changes found and indicate for which observations users should consider applying corrections to throughput or contamination rate. Field and Focus Dependence of Aperture Photometry: Small-aperture photometry is sensitive to changes to the WFPC2 Point Spread Function (PSF), which varies as function of filter, focus position, and field position. We give references to correction formulae for both field and focus dependence for the most commonly used filters. Point-Spread Functions: Progress has been made in recent years on the accurate subtraction of WFPC2 PSFs both to detect nearby faint companions of bright stellar objects and to obtain accurate stellar photometry. This document provides a short update describing the imaging dynamic range one can now expect to obtain near a bright stellar source, and our present understanding of the photometric accuracy one can obtain by PSF subtraction.

7 Introduction 3 Dithering: Dithering the telescope as a means of improving image resolution and removing detector defects has become increasingly popular among users of WFPC2 since the technique was successfully employed for the Hubble Deep Field. We present some issues related to the use of singly dithered observations (one image at each dither position), and describe how recently developed software can support such observations. The advantage of singly dithered observations is that more dither positions can be obtained in a given amount of time or number of exposures. The disadvantages are that the data are somewhat more difficult to reduce, and stellar photometry to better than a few percent cannot be guaranteed except in cases of excellent dithering coverage. A new document, the Drizzle Cookbook, presents fully worked-out examples of the analysis of dithered data. Polarization Calibration: A comprehensive model developed for the physics of polarization in WFPC2 can be used to derive a more accurate calibration of WFPC2 polarimetric observations, with an estimated accuracy of about 1.5% rms. Dark Current Change: The dark signal seen in WFPC2 images has increased by about % since launch. Under most circumstances, the dark current remains a modest contribution to the overall image noise. A new superdark has been constructed, based on 1998 data, and is currently in use in the calibration pipeline, thus ensuring correct dark current subtraction. Operating Changes: Some minor changes have been recently introduced in how WFPC2 observations are scheduled. These changes should improve the schedulability of long programs, as well as the overall efficiency of HST observations. Other Instruments: In addition to WFPC2, HST currently has another operating instrument capable of imaging, the Space Telescope Imaging Spectrograph (STIS). STIS may be preferable to WFPC2 for some imaging programs. A new instrument, the Advanced Camera for Surveys (ACS), will be installed on HST in 2000, and will be offered for observations during Cycle 10. We provide a short overview of the current and planned capabilities of these instruments, comparing them with WFPC2. WFPC2 Clearinghouse: The WFPC2 group has developed a web-based tool, the WFPC2 Clearinghouse, to allow users to easily search journal articles, STScI documentation, and user-submitted documents for topics relating to the performance, calibration, and scientific use of WFPC2. This tool is described in the hope that the information made available by its use will help observers in the preparation of their observing plans, as well as in the reduction of their data.

8 4 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 Calibration Plans: Calibration of WFPC2 continues. We provide an updated table of system efficiencies and zeropoints, which differs slightly from that in the Instrument Handbook for certain narrow-band and UV filters. We also describe the plans for future calibration of WFPC2. Users who suspect that they will have unusual or particular calibration needs should examine the calibration plan to determine if they will need to take their own calibration observations as part of their HST observing program. Photometric Anomalies To be read in conjunction with Chapter 6 of the WFPC2 Instrument Handbook, Version 4.0. Two photometric anomalies resulting from nonlinearities of the WFPC2 detectors have now been extensively characterized. The first is due to the imperfect charge transfer efficiency (CTE) of the detectors, which causes sources at high row and column numbers to appear fainter because the charge is transferred over a bigger fraction of the chip. This anomaly is increasing with time, especially for faint sources, presumably as a consequence of on-orbit radiation damage. The second, called long vs. short, causes sources to have a lower count rate - and thus appear fainter - in short exposures than in longer exposures, and appears independent of the position on the chip. The physical cause of the long vs. short anomaly is not fully understood, and it does not appear to change with time. We have developed correction formulae which appear to reduce the impact of both anomalies to about 2-3% for faint sources. Charge Transfer Efficiency During the past two years, two studies were completed resulting in a better characterization of the Charge Transfer Efficiency (CTE) problem for WFPC2, based on an analysis of observations of the globular cluster ω Cen (NGC 5139). The first study provides a set of formulae that can be used to correct for CTE loss when doing aperture photometry, based on a dataset taken on June 29, 1996 (Whitmore and Heyer 1997). 1 The second study found evidence that CTE loss for faint stars has increased with time (Whitmore, 1998). A third study (Whitmore, Heyer and Casertano 1999) is 1. Please refer to the list of References on page 45.

9 Photometric Anomalies 5 currently being prepared for publication, and will supersede the earlier work. Please check our web pages for updated information. Figure 1.1: Ratio of count rates observed for the same star (i.e., throughput ratio) as a function of the change in row position for stars in 4 different brightness ranges. The negative slope shows that a star appears brighter when it is at low row number, thus closer to the bottom of the chip and the readout amplifiers, than when it is at high row number. The effect is larger for fainter stars. See Whitmore and Heyer (1997) for details. The primary observational consequence of CTE loss is that a point source at the top of the chip appears to be fainter than if observed at the bottom of the chip, due to the loss of electrons as the star is read out down the chip (see Figure 1.1). This is called Y-CTE. There also appears to be a similar, but weaker tendency, for stars on the right side of the chip to be fainter (called X-CTE). The effects also depend on the brightness of the star and the background level. Formulae are presented in WFPC2 ISR that reduce the observational scatter in this particular dataset from 4 7% to 2 3%, depending on the filter.

10 6 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 Figure 1.2: Y-CTE loss as a function of time for different target brightness (Whitmore et al 1999). Different symbols correspond to different background levels. The straight lines represent the best-fit multilinear regression for Y-CTE as function of time, log counts and log background, as discussed in Whitmore et al (1999). A continuation of this analysis using new observations of ω Cen suggests that the CTE loss for WFPC2 is time dependent. The datasets cover the time range from April 28, 1994 (shortly after the cooldown) to February For bright stars (i.e., brighter than 200 DN when using gain = 15; equivalent to 400 DN for gain = 7) there is only a modest increase in the amount of CTE loss as a function of time. However, for faint stars the CTE loss has increased more rapidly. For example, for very faint stars (i.e., DN at a gain of 15) the CTE loss has increased from 3% to 40% for a star at the top of the chip. There is no obvious change in the value of X-CTE It should be noted that these results are based on very short (14 second) exposures, with a very low background level (the observations used in Figure 1.2 have a typical background of ~0.1 DN/pixel). Typical WFPC2 exposures are much longer than these short calibration images, resulting in

11 Photometric Anomalies 7 higher background levels, which significantly reduce the CTE loss and minimize the CTE problem for most science observations. Observers can use a number of strategies to minimize the effect of CTE loss. Longer individual exposures help by increasing both background and source counts, both of which reduce CTE loss. Users thinking of dithering may wish to take this into account if they are considering shortened exposures to allow for more dither positions. When observing a target significantly smaller than a single detector, it is advisable to place it towards the bottom of a chip; the aperture WFALL will place the target near the bottom of Chip 3. (Note however that targets larger than about 20" centered on WFALL will be split between chips, with a possible impact on photometric accuracy.) The resulting photometry can be corrected after the fact using the formulae provided in the references above; however, when the highest possible accuracy is required, another possibility is to include a special calibration observation of ω Cen, taken close to the time of the science observations and designed so as to reproduce them as closely as possible in exposure and background levels. A further possible strategy is to preflash the chip to raise the background. However, tests indicate that the required level of preflash is so high that in general more is lost than gained by this method. A variation of this is currently being tested in the noiseless preflash proposal (8450), where a flatfield exposure is read out immediately prior to a short science exposure. 2 As part of the Cycle 8 Calibration Plan, in addition to the noiseless preflash test, we will continue monitoring the CTE for point sources by repeating the key observations of ω Cen every six months (Proposal 8447). We have also added observations of a cluster of galaxies (Proposal 8456), which will yield a direct measurement of the effect of CTE for faint extended sources for more typical exposure times and background levels. The Long vs. Short Photometric Anomaly The so-called long vs. short anomaly is a nonlinearity of WFPC2 which causes the recorded count rate to increase with exposure time for a given source - the source thus appears brighter in a long exposure than in a short exposure. A recently completed study of this anomaly (Casertano and Mutchler 1998) shows that it is primarily a function of the total source counts, and, unlike the CTE anomaly, is independent of the position in the chip. In the simplest interpretation, a fraction of the total counts are lost, with the fraction decreasing as the source counts increase. The fraction lost is about 3% for a source with 300 counts, and rises to over 20% at 40 counts. Sources over 1000 counts are not measurably affected. There 2. More details can be found in Biretta and Mutchler (1998) and Whitmore (1998).

12 8 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 appears to be a weak dependence on background, in the sense that the loss of signal is slightly lower in high-background images, but this effect is not significant in terms of the overall characterization of the correction. Figure 1.3: Magnitude discrepancy for exposure times from 10s to 1000s in F814W, plotted against total measured counts. Some exposures have been preflashed with 5 to 1000 e/pixel. The major trend is against total counts. We have developed a simple correction formula that can be used to compensate approximately for the signal lost. The formula expresses the magnitude correction dm to be subtracted from the measured magnitude: dm = counts counts 2 ( 1)

13 Photometric Anomalies 9 as a function of counts, the background-subtracted source counts in a 2-pixel aperture, measured in DN at gain 7. Note that the correction formula is applied after the CTE correction discussed in the previous Section has been made. Figure 1.3 shows the uncorrected magnitude errors for individual F814W observations of a field in NGC 2419 containing stars of various magnitudes, observed with exposure times ranging from 10s to 1000s and various preflash levels. The anomaly is illustrated by the rise in magnitude errors for low source counts. The effect of our correction is shown in Figure 1.4, where the solid line and error bars plot the median and quartiles of uncorrected magnitude errors, while the dashed line indicates the median of the residual magnitude errors, after the correction in Equation (1) is applied. Magnitude errors are corrected quite well, except for very faint sources (< 30 counts). Figure 1.4: Median and quartiles of the magnitude discrepancy, before and after the correction in Equation (1) has been applied (solid and dashed lines, respectively).

14 10 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 On the basis of the evidence collected so far, the anomaly appears to be more properly a function of total counts in a stellar image, rather than a direct function of exposure time. The commonly used long vs. short name is thus somewhat of a misnomer. It should be emphasized that the correction in Equation (1) has been derived from a specific set of conditions, and may not be valid in general. Specifically, images obtained by combining several subexposures and images taken at gain 15 have not been studied. Casertano and Mutchler (1998) provide some suggestions on how to handle such cases. Further data, including a test of the noiseless preflash, will be taken as part of the Cycle 8 calibration program. The Stability of WFPC2 Photometric Calibration To be read in conjunction with Chapter 6 of the WFPC2 Instrument Handbook, Version 4.0. The long-term photometric stability of WFPC2 has been evaluated by examining the photometric monitoring data collected over a period of more than four years. Our primary standard, GRW+70D5824, has been observed roughly every four weeks, before and after decontamination procedures, both in the far UV and in the standard photometric filters. Early observations were taken monthly in both the PC and WF3; since Cycle 6, we have switched to a rotation schedule, where observations are taken in a different chip each month. Overall, a baseline of over four years is available for the PC and WF3, and about two and a half years in WF2 and WF4. The data have been analyzed and reported by Baggett and Gonzaga (1998); here we summarize their main conclusions. Overall, the WFPC2 photometric throughput, as measured via our primary standard, has remained remarkably stable throughout. Its long-term behavior in filters longward of F336W is characterized by small fluctuations (2% peak-to-peak) which appear to have no specific pattern, and there is no significant overall sensitivity trend. Aside from contamination corrections, which are only significant shortward of F555W, the same photometric zeropoints can be applied to non-uv data throughout the life of WFPC2. In contrast, the UV photometric throughput of WFPC2 has changed measurably over the years. In most cases, the throughput has increased slowly, perhaps as a result of continuing evaporation of low-level contaminants; in F170W, the best-characterized UV filter on WFPC2, the clean throughput (immediately after a decontamination) has increased in

15 Field and Focus Dependence of Aperture Photometry 11 the PC by about 9% since This behavior is not uniform, in that some UV filter/detector combinations show a modest decline in throughput (3% in F255W). Baggett and Gonzaga (1998) report the details of the secular throughput changes for the filters we monitor. Finally, the contamination rates - the rate at which the camera throughput declines after a decontamination, due to the gradual buildup of contaminants on the cold CCD windows - have generally decreased since installation of WFPC2, possibly also because the environment has become cleaner with time. (This excludes a brief period of increased contamination just after the second servicing mission.) For example, the contamination rate in F170W in the PC has decreased from 0.56%/day to 0.45%/day. Baggett and Gonzaga (1998) suggest a number of ways users can correct long-term changes in WFPC2 photometry. While these changes are generally small, users wishing to achieve high-precision photometry, especially in the UV, should follow their recommendations. Field and Focus Dependence of Aperture Photometry To be read in conjunction with Sections of the WFPC2 Instrument Handbook, Version 4.0. Photometry of point sources in WFPC2 data is most often accomplished via aperture photometry using a fairly small aperture radius (2 to 5 pixels). An aperture correction is then needed to estimate the counts outside the aperture, which often cannot be measured directly because of noise, background fluctuations, or crowding. Since the WFPC2 PSF is a function of focus, filter, and position within the field of view, the aperture correction will also change with these parameters. The changes are most significant for small apertures (< 2 pixel radius), and become negligible at apertures larger than 5 pixels. Two studies have sought to characterize the change in aperture correction under various conditions. Suchkov and Casertano (1997) discuss the variation in WFPC2 focus and its impact on the aperture correction The HST focus varies due to a combination of circumstances: long-term shrinkage of the metering truss that supports the HST secondary, compensatory moves made at approximately six-month intervals to compensate for this shrinkage, and temperature fluctuations in HST, which produce variations on time scales of hours to months. For more information, see the HST focus web site at

16 12 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 Suchkov and Casertano (1997) find that the aperture correction varies with focus by up to 10% for a 1-pixel radius in the PC, and is generally well-fitted by a quadratic function of focus position. A 10% change is measured only for 5 micron defocus, which is the largest that can be expected during normal telescope operations. The variation in aperture correction is much smaller in the WF cameras and for apertures of 2 pixels or larger, which are less sensitive to the small HST defocus. Formulae that estimate the change in aperture correction due to defocus are provided for a variety of circumstances. More recently, Gonzaga et al. (1999) have characterized the change in aperture correction as a function of filter and field position. The data are somewhat incomplete, but a clear trend is present: the aperture correction generally increases linearly as a function of distance to the chip center (see Figure 1.5). For example, the aperture correction from 1 to 5 pixel radius in the PC increases by about 0.12 mag from the chip center to the edge. Figure 1.5: Measured aperture corrections for different stars observed in the PC through filter F555W. The dominant term appears to be linear with the distance R to the chip center.

17 The WFPC2 PSF: Dynamic Range and Photometry 13 In practice, the interplay between aperture correction and defocus may be complex, since the optimal focus changes with focus and field position. A full correction has not been established, but the TinyTim PSF model (see next Section) can be used to estimate the extent of the variation in aperture correction. In general, we recommend that a minimum aperture radius of 2 pixels be used whenever possible, in order to minimize the impact of variations of the aperture correction with focus and field position; if the field is too crowded for this strategy to be used, we recommend that users verify the validity of the corrections given on a few well-exposed stars. The WFPC2 PSF: Dynamic Range and Photometry To be read in conjunction with Chapter 5 of the WFPC2 Instrument Handbook, Version 4.0. Here we supplement the Instrument Handbook with short discussions of the use of PSF subtraction to maximize image dynamic range and to obtain accurate stellar photometry. We also discuss a source of error in the published values of the HST aperture corrections. Dynamic Range The WFPC2 PSF has structure on very small scales, with significant power on scales smaller than 1 PC pixel. Thus faint objects near bright objects can be difficult to detect and to distinguish from PSF artifacts. Model PSFs (for example those produced by the TinyTim software 3 ) are quite good for many purposes, but can leave residuals as large as 10 to 20% of the peak. Recent results indicate that PSF subtraction and detection of faint objects very close to bright objects can be improved by using a composite PSF from real data, especially dithered data. Table 1.1 on page 14 indicates limits that may be obtained for well-exposed sources (nominal S/N > 10 for the faint object) where a dithered PSF image has been obtained. A technique that has been used with some success to search for nearby neighbors of bright stars is to image the source at two different roll angles, and use one observation as the model PSF for the other. In the difference image, the secondary source will appear as a positive residual at one position and a negative residual at a position separated by the change in roll 3.

18 14 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 angles. PSF artifacts generally do not depend on roll angle, but rather are fixed with respect to the telescope. Thus, small changes in the PSF between observations will not display the positive or negative signature of a true astrophysical object. Again, it is recommended that the observations at each roll angle be dithered. Table 1.1: Limiting Magnitudes for PSF Subtraction Near Bright Objects Separation in arcsec (on PC) Limiting m (without PSF subtraction) Limiting m (with PSF subtraction) Photometry PSF subtraction is also an effective means of accurate and repeatable photometry on HST. Papers presented at the 1997 HST Calibration Workshop by Remy et al. and Surdej et al. show that the subtraction of synthetic or scaled observed PSFs can be used to obtain 1 2% stellar photometry. In spite of the ability to obtain photometry through PSF subtraction, the total fraction of the light of the PSF within a given radius is not known to better than a few percent due to the difficulty of measuring the light in the faint wings of stellar PSFs (remember that there are over 17,000 PC pixels inside a radius of 3, each contributing read noise to the observation!). This difficulty has contributed to a minor error in Table 6.7 of the WFPC2 Instrument Handbook, which gives the fraction of encircled energy in the F555W filter within a 1 radius as 100%. This table is based upon the encircled energy figures from Table 2(a) of Holtzman et al. (1995a). Examination of several filters shows that about 10% of the light in the PSF is missing at 1 in the PC. Observers estimating aperture corrections for their images should be wary of this effect and note that in a later paper (Holtzman et al. 1995b) the same group normalized the HST magnitude system to the light enclosed inside of a 0. 5 radius to minimize errors caused by the uncertain aperture correction at large radii.

19 Dithering with WFPC2 15 Dithering with WFPC2 To be read in conjunction with Section 7.6 of the WFPC2 Instrument Handbook, Version 4.0 Dithering is the technique of displacing the telescope between observations either on integral pixel scales (to assist in removing chip blemishes such as hot pixels) or on sub-pixel scales (to improve sampling and thus produce a higher-quality final image). The WFPC2 Instrument Handbook provides a good introduction to dithering strategies in Section 7.6; however, our experience with processing dithered WFPC2 data has progressed substantially over the last two years, and new software tools have been introduced which make the combination of dithered data both easier and more efficient. The method we recommend is based on the variable pixel linear reconstruction algorithm, also known as drizzle (Fruchter and Hook 1997). This method has been developed into a number of tasks, incorporated into the IRAF/STSDAS packages dither and ditherii, which allow effective cosmic ray removal from singly dithered data (i.e., only one image per pointing). The dither package is part of the standard STSDAS distribution, while the ditherii package is available for download via the web at: We anticipate that ditherii will be incorporated into the standard STSDAS distribution during With this software, it is now practical to obtain high-quality images via dithering even when the available time does not permit obtaining a CR-split at each pointing, and dithering is recommended under most circumstances (subject to the cautions further below). Further information on the software in development to process dithered data can be found in two papers in the 1997 HST Calibration Workshop Proceedings: A Package for the Reduction of Dithered Undersampled Images, by Fruchter et al. (1997), and Dithered WFPC2 Images A Demonstration, by Mutchler and Fruchter (1997). Up-to-date information about dithering and related issues can also be found on the WFPC2 drizzling web site at: In order to help users reduce dithered images, we have prepared the Drizzling Cookbook (Gonzaga et al 1998), also available at the URL above. This document gives a general outline of the reduction of dithered images and provides step-by-step instructions for six real-life examples that cover a range of characteristics users might encounter in their observations. The

20 16 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 data and scripts needed to reproduce the examples are also available via the same URL. Despite all the improvements in the combination of dithered images, users should be mindful of the following cautionary notes: Processing singly dithered images can require substantially more work (and more CPU cycles) than processing data with a number of images per pointing. Removing cosmic rays from singly dithered WFPC2 data requires good sub-pixel sampling; therefore one should probably not consider attempting this method with WFPC2 using fewer than four images and preferably no fewer than six to eight if the exposures are longer than a few minutes and thus subject to significant cosmic ray flux. It is particularly difficult to correct stellar images for cosmic rays, due to the undersampling of the WFPC2 (particularly in the WF images). Therefore, in cases where stellar photometry to better than a few percent is required, the user should take CR-split images, or be prepared to use the combined image only to find sources, and then extract the photometry from the individual images, rejecting entire stars where cosmic ray contamination has occurred. Figure 1.6: On the left, a single 2400s F814W WF2 image taken from the HST archive. On the right, the drizzled combination of twelve such images, each taken at a different dither position. Offsets between dithered images must be determined accurately. The jitter files, which contain guiding information, cannot always be relied upon to provide accurate shifts. Therefore, the images should

21 Other HST Imaging 17 be deep enough for the offsets to be measured directly from the images themselves (typically via cross-correlation). In many cases, the observer would be wise to consider taking at least two images per dither position to allow a first-pass removal of cosmic rays for position determination. Finally, and perhaps most importantly, dithering will provide little additional spatial information unless the objects under investigation will have a signal-to-noise per pixel of at least a few at each dither position. In cases where the signal-to-noise of the image will be low, one need only dither enough to remove detector defects. Other HST Imaging To be read in conjunction with Section 1.2 of the WFPC2 Instrument Handbook, Version 4.0 The Space Telescope Imaging Spectrograph (STIS) offers imaging capabilities in both the optical and ultraviolet that for some limited applications provides performance superior to that of WFPC2. The interested reader should refer to the Cycle 9 Call for Proposals for a brief overview, and the updated STIS Instrument Handbook, version 3, May 1999 and Section 1.2 of the WFPC2 Instrument Handbook, version 4, for more details. Associated with this Cycle 9 Call for Proposals is a preview of the Advanced Camera for Surveys, which will be available for General Observer proposers in Cycle 10. Observers may wish to take into account the future enhancement in HST imaging capabilities promised by the Advanced Camera in order to develop a long term proposal strategy. Polarization Calibration To be read in conjunction with Section 3.5 of the WFPC2 Instrument Handbook, Version 4.0. New calibration observations have permitted a substantial improvement in the polarization calibration of WFPC2 since Cycle 6. The new results, fully described in Biretta and McMaster (1997), are based on a physical

22 Dark Current Increase 18 model of the polarization effects in WFPC2, described via Mueller matrices, which includes corrections for the instrumental polarization (diattenuation and phase retardance) of the pick-off mirror, as well as the high cross-polarization transmission of the polarizer filter. New polarization flatfields are also available. Comparison of the model against on-orbit observations of polarization calibrators shows it predicts relative counts in the different polarizer/aperture settings to 1.5% RMS accuracy. To assist in the analysis of polarization observations, we provide two Web-based utilities, available at by which users can simulate and calibrate their data. These tools have been upgraded to include effects related to the MgF 2 coating on the pick-off mirror, as well as the more accurate matrices for the cross-polarization leakage in the polarizer filter. Differences between the previous and current versions of the tools are typically around 1% in fractional polarization. Dark Current Increase To be read in conjunction with Section 4.8 of the WFPC2 Instrument Handbook, Version 4.0. Recent measurements of the dark current in the WFPC2 detectors indicate that the average level of dark current has been slowly increasing over the instrument s lifetime. Baggett et al (1998) have re-analyzed dark current measurements taken at regular intervals from 1994 to Over this five-year period, the dark current has increased by a factor of about 2.2 in the WFC CCDs and by a factor of 1.3 in the PC (values are for the center of the CCDs). A small increase in the cold junction temperatures over this time period was detected as well; however, the amount of temperature change accounts for only a very small portion of the increase in dark current. The dark current increase is smaller in the optically vignetted regions near the CCD edges, suggesting that some of the effect may be caused by increased fluorescence or scintillation in the CCD windows, rather than by the CCDs themselves. Note that the increase in dark signal we report here affects all pixels, and thus is distinct from the cyclic increase in the number of hot pixels reported in the WFPC2 Handbook. The latter are highly localized, and are almost certainly due to radiation-damaged sites on the CCD detectors. Since the dark current is generally a minor contributor to the total noise in WFPC2 images, its increase is unlikely to impact adversely the quality of WFPC2 observations, except perhaps in special cases (faint sources observed in

23 Operating Changes 19 AREA mode through narrow-band or UV filters). A new superdark has been generated from 120 dark frames taken around mid-1998 and is currently in use in the calibration pipeline; this superdark provides dark current subtraction consistent with our most recent measurements, and will be updated as needed. Operating Changes Some changes in WFPC2 operations have recently been introduced, primarily in order to increase the scheduling efficiency of HST observations in Cycle 8. First, the South Atlantic Anomaly (SAA) contours used to limit WFPC2 observations have been modified slightly. The SAA is a region where irregularities in the Earth s magnetic field cause very high cosmic ray rates. WFPC2 imaging is generally not scheduled near the SAA, so as to avoid excessive cosmic ray hits which degrade images by obliterating data in numerous pixels. These adverse effects are usually minimized by operating each instrument only when HST is outside a designated SAA avoidance contour, although WFPC2 observations of time-critical phenomena can be taken inside the SAA avoidance contour. Biretta and Baggett (1998) have analyzed available WFPC2 data, together with data from Air Force satellites flying in similar orbits, and have redefined the WFPC2 SAA avoidance contour. This change results in a 3% increase in designated SAA-free orbits, which allows better scheduling efficiency, and is expected to impact negatively less than 0.1% of WFPC2 science observations. Second, WFPC2 visits are now limited to a maximum length of 5 orbits. Very long visits (up to the previous maximum of 8 orbits) have very limited opportunities for scheduling, reduce the efficiency of telescope use, and can cause long delays in execution, with long GO wait times. The transition to shorter visits improves the scheduling opportunities for large proposals. One possible drawback is the lower pointing repeatability across visits; this is likely to be significant only for programs with special dithering requirements. A third change for Cycle 8 is that an extra minute of overhead has been added to each orbit in RPS2, which allows splitting an orbit in the phase 2 proposal into two separate spacecraft alignments. This one-minute efficiency adjustment allows much more efficient scheduling of HST orbits impacted by the SAA.

24 20 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 The WFPC2 Clearinghouse The WFPC2 Clearinghouse is a web-based tool designed to provide users with a searchable listing of all known journal articles, STScI documentation and reports, as well as user-submitted documents which report on all aspects of the performance, calibration, and scientific use of WFPC2. The Clearinghouse can be found at: The primary goal of the Clearinghouse is to make it easier for WFPC2 users to take advantage of the fact that there are hundreds of researchers reducing and analyzing WFPC2 data, and of their results. We have extensively searched through the astronomical literature and selected all articles that contain any reference or description of the calibration, reduction, and scientific analysis of WFPC2 data prior to Each article was then added to our database, with an estimate of its importance in up to 50 calibration topics. Each entry has the following format: Author: Holtzman,Mould,Gallagher, et al. Title: Stellar Populations in the Large Magellanic Cloud: Evidence for.. Year: 1997 Reference: AJ 113, 656 Science Keyword: IMF,LMC Calibration Keyword(3): psf_fitting_photometry(3) Calibration Keyword(2): bias(2) Calibration Keyword(1): photometric_zeropoint(1) Comment: Comparison of aperture and PSF fitting photometry, where the category number following each keyword stands for the following: (3)= One of the fundamental references on this topic. (2)= Some new information on this topic. (1)= General information on the subject. The user can select from a large list of WFPC2 calibration related topics (see below). The results from a Clearinghouse search will list, alphabetically by author, all articles containing references to the selected topic. For journal articles, each reference is linked to that article s entry in the ADS Abstract Database, so that users can quickly determine if that particular article is relevant to their individual needs.

25 The following topics are available: Updates to System Efficiencies and Zeropoints 21 Aperture Corrections Aperture Photometry Astrometry Bias Frames Bias Jumps Calibration Observations CCD Characteristics Charge Transfer Traps Chip-to-Chip Normalization Completeness Corrections Cosmic Rays CTE Losses Darks Data Quality Deconvolution Dithering Drizzle Field Distortion Flats Focus Hot Pixels Image Anomalies Linear Ramp Filters Long vs. Short Exposures Narrow Band Photometry Object Identification Observation Planning Photometric Transformations Photometric Zeropoint Pipeline Calibration Polarization PSF Characterization PSF Fitting Photometry PSF Subtraction Quad Filters Recalibration Red Leaks Residual Images Saturated Data Scattered Light Serial Clocks Size Measurements Software Surface Photometry SYNPHOT T=77 Observations UV Throughput Vignetting Woods Filters 1997 Servicing Mission Updates to System Efficiencies and Zeropoints To be read in conjunction with Section 6.1 of the WFPC2 Instrument Handbook, Version 4.0. Table 1.2 on page 22 is an update to Table 6.1 of the WFPC2 Instrument Handbook. New calibration information has caused us to update the estimated efficiencies and throughputs of several of the narrow-band and UV filters since the last publication of this table. These numbers are accurate to at least 10% which is sufficient for planning observations, but

26 22 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 not for the analysis of many programs. Investigators wishing to do photometry on WFPC2 images should refer to the HST Data Handbook for an explanation of the conventions used in determining WFPC2 zeropoints and should use the zeropoints given in Table 28.1 of the Data Handbook. For the most accurate and up-to-date calibrations, users should examine the on-line version of the Data Handbook to verify that no numbers of interest have changed since the last paper publication. Table 1.2: System Efficiencies and Zeropoints. Replaces Table 6.1 in the Instrument Handbook, Version 4.0 (see original table for definitions). Filter QTd λ/λ λ δ λ σ QT max dλ/dα λ p <λ> λ max m e/sec t wfsky F122M E+07 F130LP E+02 F160BW E+05 F165LP E+02 F170W E+06 F185W E+06 F218W E+06 F255W E+06 F300W E+04 F336W E+04 F343N E+06 F375N E+06 F380W E+03 F390N E+05 F410M E+04 F437N E+05 F439W E+03 F450W E+03 F467M E+04 F469N E+05 F487N E+04 F502N E+04 F547M E+03 F555W E+02

27 Updates to System Efficiencies and Zeropoints 23 Table 1.2: System Efficiencies and Zeropoints. Replaces Table 6.1 in the Instrument Handbook, Version 4.0 (see original table for definitions). Filter QTd λ/λ λ δ λ σ QT max dλ/dα λ p <λ> λ max m e/sec t wfsky F569W E+02 F588N E+04 F606W E+02 F622W E+02 F631N E+04 F656N E+04 F658N E+04 F673N E+04 F675W E+02 F702W E+02 F785LP E+03 F791W E+02 F814W E+02 F850LP E+03 F953N E+04 F1042M E+04 FQUVN-A E+05 FQUVN-B E+05 FQUVN-C E+05 FQUVN-D E+05 FQCH4N-A E+04 FQCH4N15-B E+04 FQCH4N33-B E+04 FQCH4N-C E+04 FQCH4N-D E+04 POLQ_par POLQ_per

28 24 Chapter 1: Update to the WFPC2 Instrument Handbook for Cycle 9 WFPC2 Calibration Plan To be read in conjunction with Section 8.10 of the WFPC2 Instrument Handbook, Version 4.0. Introduction In this section we discuss the Cycle 7 and 8 calibration plans for WFPC2. It is the policy of the Institute to attempt to obtain the necessary calibration files for the vast majority of user programs. In some cases, however, users may find that they will need to take calibration images as part of their program. If there is any doubt about the suitability of the present calibrations for a specific program, please feel free to contact the WFPC2 group via to help@stsci.edu. The results of the calibration programs are reported to users through the HST Data Handbook for results of general interest, and also through frequent Instrument Science Reports available from the STScI on-line information service. HST users should rely on these, rather than the Instrument Handbook, when the most accurate, up-to-date numbers are required. Overview The main goals of the WFPC2 Calibration Plans for Cycles 7 and 8 are: verify that the instrument remains stable in its main characteristics. address its photometric accuracy. follow the long-term changes that are appearing in the instrument after 5 years of on-orbit service. These goals are achieved by a mix of monitoring programs, which verify the stability and continued performance of the camera by repeating routine observations on a regular basis, and special calibrations, which have the goal to enhance the WFPC2 calibration in specific areas. Standard Monitoring Programs The stability of WFPC2 is mainly verified through the Photometric Monitoring program and the set of internal monitoring programs. Starting in Cycle 8, several of these monitors - whose execution is linked to the periodic decontaminations of the camera - have been merged into a common program together with the decontaminations themselves (programs 8441, 8459), in order to facilitate their scheduling.